BACKGROUND: A sample mix-up occurred in a tissue procurement laboratory in which aliquots of serum from two tissue donors were accidentally mislabeled. The clues to the apparent mixup involved discrepant Hepatitus C test results. In an attempt to resolve the apparent mix up, DNA typing was performed using serum samples as a possible source of genomic DNA. STUDY DESIGN AMD METHODS: Two hundred microliter aliquots of two reference sera and aliquots prepared from them were subjected to DNA extraction. PCR amplification of 9 STR loci was performed on the extracts and amplicons were analyzed by capillary electrophoresis. RESULTS: About 1 μg/ml of DNA was recovered from all serum samples and was of sufficient quality to direct the amplification of most, if not all STR loci allowing the mislabeled specimens to be traced to the proper tissue donor. CONCLUSIONS: Serum is a useful source of genomic DNA for STR analysis in situations in which such samples are the only source of DNA for testing. Interestingly, one of the tissue donors on life support and repeatedly receiving blood products, exhibited a mixed DNA profile indicative of the presence of DNA from multiple individuals in the bloodstream.